Well, I got it to fire for a millisecond and promptly sheared the flywheel key. I had noted that the key was showing signs of fatigue, and despite what I thought was adequate torquing, it let go on me. 
picture upload
I see there is one on Ebay that looks similarly worn. Also it is from the S300 motor, so I am uncertain if it is the correct dimensions. 
What type of stock can I use to fabricate a replacement ?